Asexual blood stage antigens of plasmodium falciparum which encode a rhoptry protein

ABSTRACT

DNA molecules comprising artificially constructed polynucleotide sequences substantially coresponding to all or a portion of the base sequence coding for an antigen of Plasmodium faliciparum selected from the group consisting of the Acidic Basic Repeat Antigen Rhoptry (ABRA), the antigen of any of clones Ag 169, Ag303, Ag358, Ag361, Ag372, Ag394 or Ag501, defined herein, and other antigens of P. falciparum cross-reactive therewith. Such DNA molecules are capable of being expressed as polypeptide(s). Synthetic peptides or polypeptides displaying the antigenicity of all or a portion of the above antigens of P. falciparum. Compositions for stimulating immune responses against P. falciparum antigens in a mammal, comprising at least one polyupeptide displaying the antigenicity of the above antigens of P. falciparum, together with a pharmaceutically acceptable carrier therefor.

This invention relates to synthetic peptides and polypeptides which have antigenicity suitable for providing protective immunity against Plasmodium falciparum infections, and to methods for the production thereof.

The human malaria parasite Plasmodium falciparum encodes many polypeptides that elicit an immune response in man. Recently, molecular cloning techniques have facilitated the analysis of individual polypeptide antigens that are present in this complex mixture (1). Many cDNA clones encoding these antigens have been isolated by screening Escherichia coli colonies that express the cloned sequences with human antibodies. The production and screening of these clones is described in detail in International Patent Specification No. PCT/AU84/00016.

The present invention is based upon the identification and characterisation of further asexual blood-stage antigens of P. falciparum.

According to the present invention, there is provided a DNA molecule comprising a nucleotide sequence substantially corresponding to all or a portion of a base sequence coding for one of the antigens of P. falciparum described in detail hereinafter. In particular, there is provided a DNA molecule comprising a nucleotide sequence characterized by at least a portion thereof comprising all or a portion of a base sequence shown in the accompanying Figures. Such a nucleotide sequence codes for a polypeptide comprising at least a portion which corresponds to a portion of the amino acid sequence of an antigen of P. falciparum as described herein.

The present invention also extends to synthetic peptides or polypeptides displaying the antigenicity of all or a portion of an antigen of P. falciparum as described herein, as well as to compositions for stimulating immune responses against such an antigen in a mammal, which compositions comprise at least one synthetic polypeptide displaying the antigenicity of all or a portion of the antigen, together with a pharmaceutically acceptable carrier therefor. The synthetic peptides or polypeptides according to this aspect of the invention may be prepared by expression in a host cell containing a recombinant DNA molecule which comprises a nucleotide sequence as broadly described above operatively linked to an expression control sequence, or a recombinant DNA cloning vehicle or vector containing such a recombinant DNA molecule. The synthetic peptide or polypeptide so expressed may be a fusion polypeptide comprising in addition to a portion displaying the antigenicity of all or a portion of the antigen, an additional polypeptide coded for by the DNA of the recombinant DNA molecule. Alternatively, the synthetic peptides or polypeptides may be produced by chemical means, such as by the well-known Merrifield solid-phase synthesis procedure.

(I) A RHOPTRY PROTEIN OF P. falciparum

Intraerythrocytic asexual parasites of Plasmodium falciparum are responsible for the morbidity and mortality of this serious protozoal infection of man. Propagation of the asexual parasite occurs when mature schizonts rupture and release merozoites which invade fresh erythrocytes. Invasion begins when the merozoite abuts an erythrocyte and re-orientates so that the apex of the merozoite is in contact with the erythrocyte membrane. Paired apical organelles called rhoptries discharge their contents prior to perturbation of the erythrocyte membrane and subsequent entry of the merozoite. Rhoptry proteins have been implicated as potential protective immunogens in several systems (2, 3). A cDNA clone encoding a portion of a Mr 105,000 rhoptry protein of P. falciparum has now been identified and characterised. A rhoptry protein of this molecular weight is present in several isolates of P. falciparum from widely separated geographical areas.

Several previously isolated cDNA clones expressing P. falciparum antigens contained regions of tandemly repeated peptides. It has previously been shown that these repeat regions are frequently highly antigenic and are the immunodominant regions of the molecule recognized during natural infection. The clone Ag44 expressing part of the Mr 107,000 rhoptry protein is an example where a naturally antigenic determinant is encoded by non-repeat sequence. It is not known whether other portions of this molecule contain repeat regions. The identification of this clone allows the preparation of monospecific reagents against this rhoptry protein which will enable tests of its function and potential as a protective immunogen.

Further details of the isolation and characterization of this protein will be apparent from the following detailed description, and from the accompanying figures. In the figures:

FIG. 1 shows indirect immunofluorescence of the FC27 isolate of P. falciparum asexual blood stages reacted with human antibodies against Ag44. Fluorescein staining of FIG. 1A, a late trophozoite (T) and early schizont (S) exhibiting weak fluorescence excluded over the nuclei; FIG. 1B, a mature schizont with the predominant pattern of punctate fluorescence (left) and a multiply-infected erythrocyte exhibiting both punctate and lattice patterns of fluorescence (right); FIG. 1C, punctate fluorescence associated with extracellular merozoites. Paired spots of fluorescent staining occur within a single merozoite (arrowed). Inset is of a mature schizont from an adjacent field of view.

FIG. 2 shows immunoelectron microscopy, using the protein A-gold technique, of a schizont of the FC27 isolate of P. falciparum reacted with human antibodies against Ag44. The pear-shaped rhoptries containing antigen reacting with antibodies are arrowed. (Magnified×79,000.)

FIG. 3 shows immunoblots using human antibodies affinity-purified on the fusion protein of clone Ag44. FIG. 3A--Identification of the corresponding parasite antigen in different life-cycle stages of FC27: uninfected cells (1), rings (2), trophozoites (3), schizonts (4), merozoites (5). FIG. 3B--Identification of the corresponding parasite antigen in 4 different isolates of P. falciparum grown in asynchronous culture: NF7 (1), K1 (2), FC27 (3) and V1 (4).

FIG. 4 is the nucleotide and amino acid sequence of Ag44. At the 5' end of the insert the first 2 bases derived from P. falciparum put the sequence in frame with β-galactosidase but are not translated here because the corresponding hybrid codon would not be found in the P. falciparum sequence.

FIG. 5 shows the hybridization of Ag44 cDNA to restriction fragments of P. falciparum DNA. DNA from isolates FC27 (1), K1 (2) and NF7 (3) was cleaved with EcoRI FIG. 5A and Hind 3 FIG. 5B, fractionated by electrophoresis on a 1% agarose gel, blotted to nitrocellulose, hybridized with ³² P-Ag44 cDNA and autoradiographed.

Materials and Methods (See later)

RESULTS Identification of a cDNA Encoding a Rhoptry Protein

cDNA derived from the Papua New Guinea isolate FCQ27/PNG (FC27) isolate of P. falciparum was prepared as described and inserted into the expression vector λgt11-Amp3 (1). A large number of clones expressing P. falciparum sequences were screened with human antibodies affinity purified against the FC27 isolate and seventy-eight antigen positive clones were identified (4). One such clone, Ag44, was shown to encode part of a rhoptry protein as follows. E. coli lysogenic for λAg44 were grown in liquid culture, heat-induced, lysed and coupled to CNBr-activated Sepharose. Human antibodies specific for the Ag44 fused polypeptide were affinity purified on this absorbent, and used to identify the P. falciparum protein corresponding to Ag44 by immunofluorescence and immunoblot assays.

Asynchronous cultures of the FC27 isolate were fixed to glass slides and examined by direct immunofluorescence. Proteins reactive with anti-Ag44 antibodies were localised to mature schizonts, in paired organelles within merozoites, a pattern characteristic of rhoptry proteins (FIG. 1a). Little reactivity was seen with ring forms. Several different isolate of P. falciparum: K1 from Thailand, NF7 from Ghana and V1 from Vietnam, all showed identical patterns of fluorescence.

Immunoelectron microscopy confirmed the rhoptry location of the antigen recognised by anti-Ag44 antibodies. There was heavy labelling of the pear-shaped organelles when sections of schizonts were incubated first with affinity purified human anti-Ag44 antibodies and then protein A-gold (FIG. 2).

Immunoblot analysis of lysates of synchronized P. falciparum infected cells showed that the anti-Ag44 antibodies recognised 3 closely-spaced bands of Mr 107,000, 105,000 and 103,000 (FIG. 3a). The higher molecular weight forms were more prominent in immature forms, and this may suggest a precursor product relationship. A similar set of bands was recognised when lysates of several different P. falciparum isolates were probed with anti-Ag44 antibodies (FIG. 3b).

Nucleotide Sequence of Ag44

DNA was purified from phage expressing Ag44. Only 1 insert was present and this was subcloned into the pUC and M13 vectors. The nucleotide sequence of the 494 bp R1 fragment was determined by the dideoxy method (FIG. 4). There was a long open reading frame present which extended up to nucleotide 404 and was in frame with β-galactosidase, accounting for the large fused polypeptide synthesized by λAg44 (4). The predicted amino acid sequence is displayed (FIG. 4). There are no tandemly repeated peptide elements as are commonly found in other P. falciparum antigens. The termination codon at nucleotides 405-407 presumably represents the 3' end of the coding region. This is consistent with the presence of deoxyadenosine bases present at the extreme 3' end of the DNA sequence which correspond to the poly(A) tail of the mRNA. This sequence predicted here would encode approximately 16% of the entire molecule.

Genomic Context of Ag44

DNA from three P. falciparum isolates FC27, K1 and NF7 was cleaved with EcoRI or AhaIII; size fractionated and blotted to nitrocellulose. The purified 570 bp R1 fragment of Ag44 was nick-translated and hybridized to the nitrocellulose filter. All isolates showed a common band of 1800 bp in EcoR1 digests and 5000 bp in Hind 3 digests (FIG. 5).

(II) AN ACIDIC BASIC REPEAT ANTIGEN (ABRA) OF P. falciparum

A Mr 102,000 antigen of P. falciparum, predominantly of schizonts, has been identified and characterized. Sequencing studies on 4 cDNA clones encoding parts of this antigen revealed blocks of hydrophilic dipeptide and tripeptide repeats and so the antigen has been designated the Acidic Basic Repeat Antigen (ABRA).

Further details of the isolation and characterisation of this antigen will be apparent from the detailed description hereunder, and from the accompanying figures. In the figures:

FIG. 6 shows indirect immunofluorescence of P. falciparum a sexual blood stages reacted with human antibodies to Ag196. Single fields of view for isolate V1 (FIGS. 6A and B) and FC27 (FIGS. 6C and D) examined by fluorescein (A, C) and propidium (B, D) fluorescence the erythrocytes shown contain trophozoites (T) and schizonts (S).

FIG. 7 shows immunoblots using human antibodies affinity-purified on the fusion protein of clone Ag196.

FIG. 7A. Identification of ABRA in 4 different isolates of P. falciparum grown in asynchronous culture: uninfected red cells (1), NF7 (2), K1 (3), FC27 (4) and V1 (5).

FIG. 7B. Detection of ABRA in different life-cycle stages of FC27: uninfected red cells (1), rings (2), trophozoites (3), schizonts (4) and merozoites (5).

FIG. 7C. Triton X-100--extracts of the same life-cycle stages as in (B) (lane 1 to 5). Triton-insoluble pellets were resolubilized in NaDodSO₄ : uninfected cells (6), rings (7), trophozoites (8), schizonts (9) and merozoites (10). Molecular weight markers are myosin (200 kD), β-galactosidase (116 kD), phosphorylase D (92 kD), bovine serum albumin (66 kD) and ovalbumin (45 kD).

FIG. 8A-C is the nucleotide and amino acid sequence of Ag189. The start of Ag144, Ag196 and Ag 126 in relation to Ag189 are indicated by arrows and the adjacent clone number.

FIG. 9 shows hydridization of Ag126 cDNA to restriction fragments of P. falciparum DNA. DNA from the 3 isolates of P. falciparum indicated was cleaved with EcoRI (1) and Aha III (2), fractionated by electrophoresis on a 1% agarose gel, blotted to nitrocellulose, hydridized with ³² P-Ag126 cDNA and autoradiographed. The P. falciparum isolates were: FC27 from Papua New Guinea; NF7 from Ghana and K1 from Thailand.

Materials and Methods (See later)

RESULTS ABRA is located in the mature schizont

Indirect immunofluorescence was performed on acetone-fixed, asexual blood-stage parasites using human antibodies affinity-purified on an immunoadsorbent of Ag196. The antibodies reacted strongly with erythrocytes containing schizonts and gave predominantly a lattice pattern of fluorescence which is particularly well resolved in isolate V1 (FIG. 6A). Counterstained nuclei of the developing merozoites appeared within regions that excluded fluorescein staining (FIG. 6B). Little or no reactivity was seen with ring and trophozoite stages of V1.

More intense fluorescence was observed at a given antibody dilution with isolate FC27. Staining again occurred predominantly with erythrocytes containing schizonts, but there was diffuse staining in trophozoites to a greater extent than with V1 (FIG. 6C). Fluorescence of the surface of infected erythrocytes was not seen when the assay was performed using unfixed cells or lightly glutaraldehyde-fixed and air-dried monolayers (5). Similar results with FC27 were obtained using mouse antisera against clones Ag196, Ag189, Ag126 and Ag203 belonging to the same serological family.

In immunoblots of asynchronous parasite preparations, affinity-purified human antibodies against clone Ag196 detected a dominant band of Mr 102,000, which did not vary between the 3 isolates NF7, FC27 and V1 (FIG. 7A). The corresponding protein is approximately Mr 2,000 smaller in isolate K1 from Thailand (FIG. 7A).

In immunoblots of life-cycle stages (FIG. 7B) the dominant Mr 102,000 band was present in schizonts was poorly represented or absent from other stages. A weak band of Mr 230,000 was also present in schizont preparations (FIG. 7B). The target antigen was recovered in Triton extracts of infected erythrocytes and no additional material was detected by anti-Ag196 antibodies when pellets were resolubilized in NaDodSO₄ sample buffer (FIG. 7C).

Nucleotide and amino acid sequence

The cDNA inserts of 4 members of the Ag196-family were isolated. The insert of Ag189 was subcloned into the vector M13mp8 and its nucleotide sequence determined by the dideoxy procedure. Ag189 contains an insert of 965 bp, which has a single open reading frame extending through the whole cDNA. This frame is shown in FIG. 8. All the other frames are interrupted by multiple stop codons. Ag189 is not in frame with β-galactosidase and does not produce a large fused polypeptide. A number of other clones from similar expression libraries were out of phase with β-galactosidase (6).

Hydrophilic dipeptide and tripeptide repeats predicted from the sequence of ABRA

The sequence of Ag189 from position 1 to 834 encodes predominantly hydrophilic amino acids. At the 3' end starting at position 835 extends a highly charged region which consists of 10 dipeptide repeats (Glu-Lys) and 6 interspersed tripeptide repeats (Glu-Glu-Lys). The repeat-block is flanked on either side by three glutamic acids.

Three blocks of 12 nucleotides starting from position 678 to 714 exhibits a high degree of homology. These "cryptic" dodeca-nucleotide repeats only show a minor degree of similarity on the amino acid level. Asparagine and isoleucine in position 3 and 4 in the first repeat appear again in the same position in the third repeat and glutamines were found in position 1 and 2 of the second and the third cryptic repeat.

A dodecapeptide was synthesized comprising the amino acid sequence Glu-Lys-Glu-Glu-Lys-Glu-Lys-Glu-Glu-Lys-Glu-Lys and the binding of antibodies in malarial sera from PNG to this peptide was tested by a radioimmunoassay (RIA). The malarial sera gave no signal in the RIA. This result was surprising because synthetic peptides corresponding to six other repeating sequences that have been determined in other antigens of P. falciparum all gave positive results (7, 8, 15).

In order to exclude sequencing errors the complete nucleotide sequence of Ag189 was again determined and an identical sequence and reading frame was obtained. In addition, the inserts of 3 further clones were sequenced, namely Ag126, Ag144 and Ag196 coding for segments of the same P. falciparum. These 3 clones are all in phase with β-gal, produce large fused polypeptides and exhibit the same open reading frame as Ag189. Therefore there is certainty about the reading frame. The sequence of Ag126, 144 and 196 includes in all 3 clones the region with the block of di- and tripeptide repeats. However differences among the 4 cDNA clones were also noted.

Ag144 which is 581 bp long commences at position 387 in relation to Ag189 and has deleted 6 bp in position 950 to 955, but contains 7 additional As at the 3' end of the cDNA, which codes for two more lysines. Ag126 and Ag196 are 451 and 452 bp long respectively and both start at position 458 in relation to Ag189. The Ag126- and Ag196-insert exhibit a deletion extending from position 901 to 955 in the sequence shown in FIG. 8. It is believed that these deletions are artefacts of clonging in M13. Similar problems of maintaining cDNA inserts in M13 have been observed with other malarial antigens (15).

Ag126 and Ag196 both differ from the sequence of Ag189 and Ag144 in two nucleotides. Ag126 and Ag196 contain at position 461 (in relation to Ag189) a "T" instead of an "A", replacing tyrosine by phenylalanine and in position 806 a "T" instead of "C", which has no effect at the amino acid level. Ag126 and Ag196 have 3 and 4 additional As at the 3' end coding for 1 and 2 more lysines, respectively.

Genomic organisation of ABRA

The insert of Ag126 was used in Southern blot experiments to investigate the genomic organisation of ABRA. DNA from 3 geographical isolates of P. falciparum, the homologous strain FC27 from Papua New Guinea, NF7 from Ghana and K1 from Thailand were restricted with EcoRI and AhaIII, size-fractionated on 1% agarose gels, blotted on nitrocellulose and probed with the ³² p-labelled insert of Ag126. As can be seen in FIG. 9, the insert hybridized to a single 6.4 kb EcoRI fragment and a 1 kb AhaIII fragment in each isolate investigated. In addition, the DNAs of a further 3 isolates from Papua New Guinea (IMR143, IMR144 and MAD71) were probed with the 581 bp insert of Ag144 and showed identical fragment sizes in these isolates (data not shown).

III OTHER ANTIGENS CLONED IN E. coli

Several other antigens of P. falciparum, which are natural immunogens in man (and therefore potential vaccine candidates), have been identified with antibodies raised against or affinity purified on P. falciparum antigens expressed from cDNA sequences cloned in E. coli using the λAmp3 vector. The clones, and the apparent molecular weights and stage specifities (determined by immunofluorescent microscopy) of the corresponding parasite antigens, are listed in Table 1.

FIG. 10A-B is the nucleotide sequence of clone Ag169;

FIG. 11 is the nucleotide sequence of clone Ag303;

FIG. 12 is the nucleotide sequence of clone Ag358;

FIG. 13A-B is the nucleotide sequence of clone Ag361;

FIGS. 14A-D show the nucleotide sequence of clone Ag394; and

FIG. 15 shows indirect immunofluorescence on acetone-methanol fixed bloodstages of P. falciparum reacted with antibodies directed against antigens produced by Ag501 in bacteria T.--trophozoite (minimal reaction). S-schizont. G.--gametocyte (no reaction seen).

                  TABLE 1                                                          ______________________________________                                                   Corresponding P. falciparum Antigen                                                            Predominant                                                      Apparent Molecular                                                                           Location by                                          Clone       Weight (Mr)*  Immunofluorescence                                   ______________________________________                                         Ag169       N.A.          N.A.                                                 Ag303(Ag331)**                                                                             125,000-130,000                                                                              Schizonts                                            Ag358       Dominant bands are                                                                           All stages                                                       210,000; 190,000                                                               and 140,000                                                        Ag361       70,000        Mature stages                                        Ag372       195,000; 140,000                                                                             Mature stages                                                    and 80,000                                                         Ag394       140,000***    All stages including                                                           rhoptry locations.                                   Ag501       ˜130,000                                                                               Mature stages                                        ______________________________________                                          N.A.  Not available                                                            *The apparent molecular weights (Mr) have been determined by Western           blotting from 7.5% gels using antigens from the FC27 P. falciparum             isolate. In some cases, the Mr can vary considerably in other isolates an      of other gel conditions are employed. Also in some cases numerous other        weaker bands are seen, presumably reflecting breakdown products or             crossreactions.                                                                **It has been found that Ag303 and Ag331 correspond to fragments of the        one coding sequence.                                                           ***Crossreactions with Ag23 and with bands of 105,000 and 102,000 were         also observed.                                                           

MATERIALS AND METHODS Parasites

P. falciparum isolates FCQ27/PNG (FC27), IMR143, IMR144 and MAD71 were obtained through the Papua New Guinea Institute of Medical Research. NF7 from Ghana, and K1 from Thailand, were obtained from D. Walliker, Edinburgh University. V1 from Vietnam was obtained from L. Miller, National Institute of Health, Bethesda, U.S.A. Parasites were maintained in asynchronous in vitro culture in Group O human erythrocytes according to Trager and Jensen (9). To obtain stage-specific life-cycle forms, parasite cultures were synchronised twice to within a six hour spread of maturation using sorbitol (10) and harvested at various time points of the asexual cycle. Naturally released merozoites were obtained as described previously (11).

Sera

Sera were obtained with informed consent from individuals living in the Madang region of Papua New Guinea. Some patients presented with acute malaria while in others, asymptomatic parasitaemia was detected in the course of routine surveys. Parasitaemic individuals were treated with chloroquine and convalescent serum was collected one or two weeks later. Parental consent was obtained before taking samples from children. In all cases, serum was separated and stored at -20° C. for up to 12 months then held at -70° C. Presence or absence of splenomegaly was documented for some subsets and parasiteaemia was assessed from a thick blood smear in all cases.

Clones Expressing P. falciparum Antigens

Methods for construction of the P. falciparum cDNA expression library and isolation of clones by antibody screening have been published (1). Replicas of the antigen-positive clones were grown overnight at 30°, induced at 38°, and lysed in situ as described (12). Individual human sera were pretreated to remove anti-E. coli activity, reacted with the colonies at a final dilution of 1:500 in 3% bovine serum albumin/Tris saline, pH 9.6 albumin, and the colonies then reacted with ¹²⁵ I protein A from Staphylococcus aureus and autoradiographed as described (12).

HYBRIDIZATION EXPERIMENTS

DNA carrying inserts were purified by CsCl centrifugation, digested with EcoRI, end-labelled with ³² P-dATP by the Klenow fragment of DNA polymerase I and size-fractionated on a 1% low-melting agarose-gel. The labelled inserts were recovered and hybridized to the bank of antigen-positive clones. In some cases the insert was first subcloned in the plasmid pUC-9 (13) purified by gel electrophoresis and then nick translated. Inserts which had been subcloned in this way were used in Southern blot experiments. For Southern blots, two micrograms of parasite DNA was digested with restriction, endonuclease according to the manufacturer's instructions, electrophoresed in a 1% agarose gel and blotted to nitrocellulose filters which were then hybridised with 10⁶ cpm/ml of the various probes.

NUCLEOTIDE SEQUENCE DETERMINATION

The dideoxy chain termination method (14), was employed for sequence determination. The inserts of the various antigen-expressing clones and fragments generated by digestion with appropriate restriction endonucleases were cloned onto M13mp8 and/or M13mp9 (13).

AFFINITY PURIFICATION OF HUMAN ANTIBODIES AGAINST CLONED MALARIA ANTIGENS

Induced 50 ml cultures of antigen positive clones were prepared as described previously (15). The pelleted bacteria were sonicated and soluble proteins were conjugated to CNBr-activated Sepharose (Pharmacia, Sweden). Antibodies from a pool of human plasma were affinity-purified on the immobilised antigen as described (15).

INDIRECT IMMUNOFLUORESCENCE

Thin blood films of parasitized erythrocytes from asynchronous cultures of P. falciparum were fixed in 90% acetone/10% methanol and reacted with affinity-purified human antibodies. Sera from mice immunized with bacterial lysates of antigen-positive clones were also examined (16). Fluorescein-conjugated sheep anti-human Ig or sheep anti-mouse Ig antisera were used as the second antibody. Parasite nuclei were counterstained with propidium iodide and the slides were mounted in 90% glycerol/10% PBS containing p-phenylenediamine for viewing under U.V. illumination.

IMMUNOELECTRON MICROSCOPY

Parasitized erythrocytes were fixed with glutaraldehyde, sectioned after being embedded in L. R. White resin and incubated with appropriately diluted antibodies and protein A-gold using published procedures (11).

IMMUNOBLOTTING

Merozoites and infected erythrocytes containing either stage-specific or asynchronous parasites were diluted in sample buffer containing 3% SDS, 62.5 mM Tris-HCl, β-mercaptoethanol, pH 6.8 and heated for 2 min at 100° C. After centrifugation at 12,000 g for 10 min., protein extracts were fractionated on 7.5% or 10% polyacrylamide/SDS gels and transferred electrophoretically to nitrocellulose. Filters were blocked with 5% non-fat milk powder in phosphate-buffered saline (PBS) pH 7.4 and reacted with affinity purified human antibodies. They were then incubated in ¹²⁵ I-labelled protein A and autoradiographed.

In a separate experiment, parasitized cells and merozoites were first incubated in PBS containing 0.5% Triton X-100, 5 mM PMSF, 1 mM TPCK, 2.5 mM EDTA and 2 mM iodo-acetamine for 30 mins at room temperature and centrifuged at 12,000 g for 10 min. Supernatants and pellets were then individually diluted to equivalent final volumes in sample buffer and treated as before.

REFERENCES

1. Kemp, D. J., Coppel, R. L., Cowman, A. F., Saint, R. B., Brown, G. V. and Anders, R. F. (1983) Proc. Natl. Acad. Sci. USA 80, 3787-3791.

2. Holder, A. A. and Freeman, R. R. (1981) Science, 193, 673-676.

3. Perrin, L., Chizzolini, C., Lebon, H., Shaw, A., Merkli, B. and Stocker, J. In "Proc. Asia & Pacific Conference on Malaria", Honolulu, Hawaii, USA, Apr. 21-27, 1985.

4. Anders, R. F., Coppel, R. L., Brown, G. V., Saint, R. B., Cowman, A. F., Lingelbach, K. R., Mitchell, G. F. and Kemp, D. J. (1984) Molec. Biol. Med. 2, 177-191.

5. Perlmann, H., Berzins, K., Wahlgren, M., Carlsson, J., Bjorkmann, A., Patarvoyo, M. E. and Perlman, P. (1984) J. Exp. Med. 159, 1686-1704.

6. Dame, J. B., Williams, J. L., McCutchan, T. F., Weber, J. L., Wirtz, R. A., Hockmeyer, W. T., Sanders, G. S., Reddy, E. P., Maloy, W. L., Haynes, J. D., Schneider, I., Roberts, D., Diggs, C. L. and Miller, L. H. (1984) Science 225, 593-599.

7. Coppel, R. L., Cowman, A. F., Lingelbach, K. R., Brown, G. V., Saint, R. B., Kemp, D. J. and Anders, R. F. (1983) Nature (London) 306, 751-756.

8. Coppel, R. L., Cowman, A. F., Anders, R. F., Bianco, A. E., Saint, R. B., Lingelbach, K. R., Kemp, D. J. and Brown, G. V. (1984) Nature (London) (in press).

9. Trager, W. and Jensen, J. B. (1976) Science 193, 673-676.

10. Lambros, C. and Vanderberg, J. P. (1979) J. Parasitol 65, 418.

11. Brown, G. V., Culvenor, J. G., Crewther, P. E., Bianco, A. E., Coppel, R. L., Saint, R. B., Stahl, H-D, Kemp, D. J. and Anders, R. F. (1985) J. Exp. Med 162, 774-779.

12. Stahl, H-D., Coppel, R. L., Brown, G. V., Saint, R. B., Lingelbach, K., Cowman, A. F., Anders, R. F. and Kemp, D. J. (1984) Proc. Nat. Acad. Sci. USA 81, 2456-2460.

13. Messing, J. and Vieira, J. (1982) Gene 19, 269-276.

14. Sanger, R., Nicklen, S. and Coulson, A. R. (1977) Proc. Nat. Acad. Sci. USA 74, 5463-5467.

15. Stahl, H-D., Crewther, P. E., Anders, R. F., Brown, G. V., Coppel, R. L., Bianco, A. E., Mitchell, F. G. and Kemp, D. J. (1985) Proc. Nat. Acad. Sci. USA 82, 543-547.

16. Coppel, R. L., Brown, G. V., Mitchell, G. F., Anders, R. F. and Kemp, D. J. (1984) EMBO J. 3, 403-407. 

We claim:
 1. An isolated and purified DNA molecule comprising: a nucleotide sequence encoding an antigen of P. falciparum selected from the group consisting of:a M_(r) 103,000 Rhoptry Protein, a M_(r) 105,000 Rhoptry Protein, and a M_(r) 107,000 Rhoptry Protein.
 2. A DNA molecule according to claim 1 which is the nucleotide sequence shown in FIG. 4 or a portion thereof encoding an antigenic peptide.
 3. A DNA molecule comprising: a recombinant DNA molecule containing a nucleotide sequence capable of expressing an antigen of P. falciparum selected from the group consisting of:a M_(r) 103,000 Rhoptry Protein, a M_(r) 105,000 Rhoptry Protein, and a M_(r) 107,000 Rhoptry Protein.
 4. A DNA molecule according to any one of claims 1 to 3, operatively linked to an expression control sequence.
 5. A DNA vector comprising: a nucleotide sequence of any one of claims 1 to 3 or a portion thereof encoding an antigenic peptide, wherein said sequence is operatively linked to an expression control sequence.
 6. A DNA vector comprising: a nucleotide sequence of any one of claims 1 to 3 or a portion thereof encoding an antigenic peptide, wherein said sequence is operatively linked to an expression control sequence and inserted into a bacteriophage.
 7. A host cell containing a DNA molecule according to claim
 4. 8. A host cell containing a DNA vector according to claim
 5. 